Central U.S. abnormality in climate change and its response to global warming

The climate system is complex and involves many intertwined, interactive processes. Consequently, the direction and magnitude of climate change is a result of the various positive and negative feedbacks. On a regional scale, local climate change also reflects remote forcing and teleconnection patterns. Diagnosing individual climate change feedbacks will improve the understanding of climate dynamics and shed light on separating climate change into natural and anthropogenic components. This project proposes feedback processes and examines their contribution to abnormal climate change in the central and eastern U.S., which experienced a cooling trend is past decades.

The first process to be examined is baroclinicity, where the horizontal gradient in surface warming increases thermal wind and baroclinic instability, which then further interacts with climate change. The second process is soil moisture feedback. Climate change causes soil moisture to change, which alters the soil heat capacity and thus causes a feedback on nearsurface temperature changes. The last process is the planetary boundary-layer (PBL) depth/lowlevel jet (LLJ) feedbacks. A stronger surface warming and thus a higher PBL height upstream produce a stronger nocturnal LLJ and moisture transport downstream, generating an increase in cloudiness leading to a subsequent cooling.

Given the processes outlined above, the objectives of our proposal are: 1) to determine to what extent the above regional feedbacks contribute to the unusual summer cooling in the central and eastern U.S.; 2) to assess if these feedbacks can help explain why most IPCC AR4 GCMs were unable to reproduce the cooling trends in their 20th century historic simulations; 3) to project if this cooling trend will continue in coming decades; and 4) examine if these feedback processes also contribute to similar local cooling documented over other continents. Address these issues will help determine whether the abnormal central and eastern U.S. climate change is a regional response to (i.e., a result of) global warming, suggesting that the cooling trend would continue in lock-step with global warming, or it is related to some other transient processes, meaning that the central and eastern U.S. would be much warmer if these processes disappear in the future.

To achieve our objectives, a series of numerical experiments will be carried out using the global NCAR Weather Research and Forecast (WRF) model and Climate WRF (CWRF) with enhanced land surface and boundary layer schemes. Statistical tools including canonical correlation analysis (CCA), principle component analysis (PCA), and factor separation will also be used to diagnose the associations between these feedbacks and anomalous mid-continent cooling.